Question

21. An object starting from rest travels 20 m in first 2 s and 160 m in next 4 s. What will be the velocity after 7 s from the start.​

Answer 1

From s=ut+21at2

In first 2 s,

s=20m

20=21a(2)2=2a

a=10 m/s2

Velocity at the end of 2 s is 

v=u+at=0+10×2=20m/s

In next 4s, s=ut+21a′t2

160=20×4+21a′(4)2

80=8a′

a′=10 m/s2

 It shows that acceleration is uniform. From v=u+at

v=0+10×7=70 m/s 

Answer: 70m/s